This invention relates to air depolarized electrochemical cells. This invention is relates specifically to metal-air, air depolarized electrochemical cells, especially elongate cylindrical cells. Elongate cells are described herein with respect to cells having the size generally known as xe2x80x9cAA.xe2x80x9d
Button cells, also illustrated herein, are commercially produced in smaller sizes having lesser height-to-diameter ratios, and are generally directed toward use in hearing aids, and computer applications. Such button cells generally feature overall contained cell volume of less than 2 cm3, and for the hearing aid cells less than 1 cm3.
The advantages of air depolarized cells have been known as far back as the 19th century. Generally, an air depolarized cell draws oxygen from air of the ambient environment, for use as the cathode active material. Because the cathode active material need not be carried in the cell, the space in the cell that would have otherwise been required for carrying cathode active material can, in general, be utilized for containing anode active material.
Accordingly, the amount of anode active material which can be contained in an air depolarized cell is generally significantly greater than the amount of anode active material which can be contained in a 2-electrode cell of the same overall size. By xe2x80x9c2-electrodexe2x80x9d cell, we mean an electrochemical cell wherein the entire charge of both anode active material and cathode active material are contained inside the cell structure when the cell is received by the consumer.
Generally, for a given cell size, and similar mass, an air depolarized cell can provide a significantly greater number of watt-hours of electromotive force than can a similarly sized, and similar mass. 2-electrode cell using the same, or a similar, material as the anode electroactive material.
Several attempts have been made to develop and market commercial applications of metal-air cells. However, until about 1970""s, such cells were prone to leakage, and other types of failure.
In the 1970""s, metal-air button cells were successfully introduced for use in hearing aids, as replacement for 2-electrode cells. The cells so introduced were generally reliable, and the incidence of leakage had generally been controlled to an extent sufficient to make such cells commercially acceptable.
By the mid 1980""s, zinc-air cells became the standard for hearing aid use. Since that time, significant effort has been made toward improving metal-air hearing aid cells. Such effort has been directed toward a number of issues. For example, efforts have been directed toward increasing electrochemical capacity of the cell, toward consistency of performance from cell to cell, toward control of electrolyte leakage, toward providing higher voltages desired for newer hearing aid appliance technology, toward higher limiting current, and toward controlling movement of moisture into and out of the cell, and the like.
An important factor in button cell performance is the ability to consistently control movement of the central portion of the cathode assembly away from the bottom wall of the cathode can during final cell assembly. Such movement of the central portion of the cathode assembly is commonly known as xe2x80x9cdoming.xe2x80x9d
Another important factor in button cell performance is the electrical contact between the cathode current collector and the cathode can or other cathode terminal. Conventional cathode current collectors comprise woven wire screen structure wherein ends of such wires provide the electrical contact between the cathode current collector and the inner surface of the cathode can.
While metal-air button cells have found wide-spread use in hearing appliances, and some use as back-up batteries in computers, air depolarized cells have, historically, not had wide-spread commercial application for other end uses, or in other than small button cell sizes.
The air depolarized button cells readily available as items of commerce for use in hearing aid appliances are generally limited to sizes of no more than 0.6 cm3 overall volume. In view of the superior ratio of xe2x80x9cwatt-hour capacity/massxe2x80x9d of air depolarized cells, it would be desirable to provide air depolarized electrochemical cells in additional sizes and configurations, and for other applications. It would especially be desirable to provide air depolarized electrochemical cells which are relatively much larger than button cells. For example, it would be desirable to provide such cells in xe2x80x9cAAxe2x80x9d size as well as in the standard button cell sizes.
It is an object of the invention to provide an air depolarized electrochemical cell wherein a stop ledge in a top grommet faces a stop groove in a cathode can, preventing the grommet from being pushed too far inwardly into the cell.
It is another object to provide a cell wherein the cathode assembly can extend into a slot at an outer edge of the top grommet.
Another object of the invention is to provide a top grommet having an upwardly extending leg overlain by a top closure member.
Yet another object is to provide a tubular air depolarized cell wherein the air diffusion member controls the rate of entry of air to the reaction surface, for example as a function of density of the air diffusion member, whereby the air diffusion member controls the limiting current of the cell.
Still another object of the invention is to provide an air depolarized cell which is free from adhesive bonding of the separator to the cathode assembly.
It is yet another object to provide an air depolarized cell wherein the bottom of the cathode current collector is in a bottom slot, in a path of flow of current between a reaction surface of the cathode assembly and a positive electrode terminal.
A yet further object is to provide a method of fabricating cells, including crimping a top washer and an outer leg of a top grommet at the same time, including closing a slot defined between legs of the grommet.
In a first family of embodiments, the invention comprehends a tubular air depolarized electrochemical cell, having a length, a top and a bottom. The cell comprises a cathode, including a cathode can and a tubular air cathode assembly extending along the length of the cell; an anode, including electroactive anode material in an anode chamber disposed inwardly, of the cathode assembly, in the cell; a separator between the anode material and the cathode assembly; electrolyte dispersed in the anode, cathode, and separator; bottom closure structure closing the bottom of the cell; and top closure structure closing the top of the cell. The top closure structure can comprise a top grommet having an outwardly extending step having a downwardly directed surface disposed about a perimeter of the top grommet, and an inwardly extending ledge on the cathode can having an upwardly directed surface disposed about a perimeter of the cathode can. The downwardly directed surface of the outwardly extending step abuts the upwardly directed surface of the inwardly directed ledge. The abutment of the step and the ledge cooperatively prevents movement of the top grommet, from the described location, downwardly toward the bottom closure structure.
Some embodiments include a seal extending into a slot between the grommet and a top closure member of the cell. The air cathode assembly, for example a cathode current collector or air diffusion member, or both, can extending into the slot, the seal being disposed between the grommet and the cathode assembly. The air diffusion member can extend into the slot from an outer surface of the cathode assembly, extending about an upper edge of the cathode current collector, and downwardly toward an inner surface of the cathode assembly such as toward the cathode current collector.
In some embodiments, the air cathode assembly extends into the slot along a longitudinally-extending length of the slot, the seal extending, in the slot, along substantially the entirety of the length of the slot occupied by the cathode current collector.
In some embodiments, the seal extends upwardly into the slot from an outer surface of the cathode assembly, about an upper edge of the cathode current collector, and downwardly along an inner surface of the cathode current collector, and thereby lines substantially the entirety of that portion of the slot which resides between the grommet and the top closure member and which is occupied by the cathode current collector.
In preferred embodiments, the grommet is confined to a top portion of the air depolarized electrochemical cell.
In some embodiments, a top portion of the cell has an outer perimeter thereabout below the top of the cell, the top closure member is crimped at a first locus against the grommet at the slot and is further crimped against the grommet at a second locus displaced longitudinally from the first locus, about the outer perimeter between the slot and the top of the air depolarized electrochemical cell, thus to provide two longitudinally-spaced crimps about the top of the cell.
In some embodiments, the cathode assembly further comprises an air diffusion member disposed outwardly in the cathode assembly and extending upwardly into the slot, as the seal, between the grommet and the top closure member of the cell.
In preferred embodiments, the seal comprises at least two layers of an air permeable microporous sheet material, wrapped continuously and without intervening end, to form an outer surface of the cathode assembly, the seal comprising the at least two layers in the slot.
In some embodiments, at least a portion of the air cathode assembly is openly exposed as an outer surface to the ambient environment, and optionally the air depolarized cell comprises a bottom closure member separate and distinct, and spaced from, a top closure member, the cell being substantially devoid of enclosing (e.g. cathode) can structure along a substantial portion of the length of the cell.
A second family of embodiments comprehends a tubular air depolarized cell, having a length, a top, and a bottom. The cell comprises a cathode, including an air cathode assembly extending along the length of the cell; an anode, including electroactive anode material disposed inwardly in the cell, of the cathode assembly; a separator between the anode material and the cathode assembly; electrolyte dispersed in the anode, cathode, and separator; bottom closure structure closing the bottom of the cell; and top closure structure closing the top of the cell. The top closure structure comprises a top grommet having an upwardly extending inner leg and a downwardly extending outer leg, defining a slot about an outer perimeter of the top grommet, and a top closure member extending downwardly over the outer leg of the top grommet and terminating in a top portion of the air depolarized cell, the air cathode assembly extending upwardly into the slot.
Typically, the top closure member in these embodiments has a crimping bias urging together the inner and outer legs of the top grommet, and thereby applying a crimping bias against the cathode assembly in the slot.
In a third family of embodiments, the invention comprehends a tubular air depolarized electrochemical cell, having a length, a top, and a bottom. The cell comprises a cathode including a cathode assembly extending along the length of the cell, the cathode assembly comprising a catalytically active material between a cathode current collector and an outwardly-disposed air diffusion member; an anode, including electroactive anode material disposed inwardly in the cell, of the cathode assembly; a separator between the anode material and the cathode assembly; electrolyte dispersed in the anode, cathode, and separator; bottom closure structure closing the bottom of the cell: and top closure structure closing the top of the cell. The top closure structure comprises a top grommet having an upwardly extending leg, the air cathode assembly extending upwardly adjacent an outer surface of the leg of the air grommet, and a top closure member overlying the leg of the top grommet, extending over a top of the cathode assembly, and extending downwardly in surface-to-surface contact with the air diffusion member.
In some embodiments, the top grommet further comprises an outer flange extending outwardly from a top of the upwardly extending leg, the outer flange extending over a top edge of the cathode current collector and over a top of the diffusion member.
In some embodiments, the top closure member overlies the outer flange of the grommet and extends thence downwardly into contact with the diffusion member.
In some embodiments, the outer flange of the top grommet is disposed between the top closure member and a top edge of the cathode current collector.
In some embodiments, the air diffusion member is folded over a top edge of the cathode current collector, and extends downwardly between the cathode current collector and the upwardly extending leg of the top grommet.
The invention further comprehends a method of fabricating a tubular air depolarized electrochemical cell having a top, a bottom, a positively-charged cathode including an air cathode assembly, a negatively-charged anode including an anode cavity containing electroactive anode material, a separator between the electroactive anode material and the cathode assembly, electrolyte dispersed in the anode material, cathode assembly, and separator, and a bottom closure member electrically connected to the cathode assembly. The method comprises assembling a tubular cathode assembly to a bottom closure member and placing a separator inwardly of the cathode assembly, and thereby creating a separator/cathode assembly combination and defining the anode cavity inwardly of the separator; loading electroactive anode material into the anode cavity; and assembling a top grommet to a top portion of the separator/cathode assembly combination such that an outer surface of an upwardly-extending inner leg of the top grommet is juxtaposed against an inner surface of an upper portion of the separator/cathode assembly combination, and an outer flange or leg of the top grommet extends outwardly from a top edge of the separator/cathode assembly combination, and outwardly from both the inner leg and the top edge of the separator/cathode assembly combination such that the outer leg is spaced from an outer surface of the cathode assembly. The method further comprehends assembling a top closure member over the top grommet, the top closure member having an outer leg extending over the outer leg of the grommet; and crimping the top closure member inwardly toward the outer surface of the cathode assembly, and correspondingly crimping the outer leg of the top grommet into securing surface-to-surface contact with the outer surface of the cathode assembly.
In some embodiments, prior to crimping, the outer leg of the top grommet is spaced from the cathode assembly by an angle xcex2 of about 2 degrees to about 90 degrees, preferably an angle xcex2 of about 2 degrees to about 30 degrees.
In some embodiments, at least a portion of the air cathode assembly in the fully fabricated cell is openly exposed as an outer surface to the ambient environment.
In some embodiments, in the fully fabricated cell, the bottom closure member is separate and distinct, and spaced from, the top closure member, the cell being substantially devoid of enclosing can structure along a substantial portion of the length of the cell.
The invention still further comprehends embodiments wherein a tubular air depolarized electrochemical cell, has a length, a top, and a bottom. The cell comprises a cathode including a cathode assembly extending along the length of the cell, the cathode assembly comprising a catalytically active material between a cathode current collector and an outwardly-disposed air diffusion member; an anode, including electroactive anode material; a separator between the anode material and the cathode assembly; electrolyte dispersed in the anode, cathode, and separator; bottom closure structure closing the bottom of the cell; and top closure structure closing the top of said cell. Cathodic oxygen passes through the air diffusion member in order to reach a reaction surface of the catalytically active material. The air diffusion rate capacity of the air diffusion member provides air to the reaction surface at a rate less than required to support a maximum reaction rate of reactants supplied by other reaction materials at the cathode reaction surface, whereby the air diffusion member controls a limiting current of the cell.
In preferred embodiments the air diffusion member extends along a substantial portion of the length of the cell, openly exposed as an outer surface of the cell, to the ambient environment, the bottom closure member preferably being separate and distinct, and spaced from, the top closure member.
A further family of embodiments comprehends a tubular air depolarized electrochemical cell, having a length, a top, and a bottom. The cell comprises an air cathode including a tubular air cathode assembly extending along the length of the cell; an anode, including electroactive anode material in an anode cavity; a separator between anode material and air cathode assembly: electrolyte dispersed in the anode, cathode, and separator, the separator being in surface-to-surface contact with the tubular air cathode assembly, the air depolarized cell being free from adhesive bonding the separator to the cathode assembly.
In some embodiments, the cell includes bottom closure structure closing the bottom of the cell. The bottom closure structure defines a slot, receiving a bottom portion of the cathode assembly, fixedly holding the cathode assembly in the slot.
In preferred embodiments, the separator has a bottom edge disposed adjacent and above the slot.
Yet further, the invention comprehends an air depolarized electrochemical cell, having a top and a bottom, and a transverse cross-section disposed along a length of the cell. The cell comprises a cathode including an air cathode assembly extending along the length of the cell; an anode, including an anode cavity, and electroactive anode material in the anode cavity; a separator between the anode material and the cathode assembly; electrolyte dispersed in the anode, cathode, and separator; bottom closure structure comprising a bottom wall of the cell, and closing the bottom of the cell; top closure structure closing the top of the cell. A slot extends downwardly adjacent an outer side wall of the bottom closure member, the cathode assembly extending downwardly into the slot and making electrical connection with the bottom closure member in the slot, the bottom closure member being in a path of flow of electric current between a reaction surface of the cathode assembly, and a positive electrode terminal of the cell.
Some embodiments include an electrically insulating bottom seal member extending generally across that portion of the transverse cross-section which spans the anode cavity, the bottom seal member separating the electroactive anode material from the bottom wall, and providing a seal about the side wall of the anode cavity at the separator, the separator extending downwardly below the top of the insulating bottom seal member, and terminating above a bottom edge of the cathode current collector.
In preferred embodiments, at least a portion of the air cathode assembly is openly exposed as an outer surface to the ambient environment, optionally with the bottom closure structure separate and distinct, and spaced from, the top closure member, the electrochemical cell being substantially devoid of enclosing can structure along a substantial portion of the length of the cell.